578 THE INVERTEBRATA 



The majority of the members of this class are extinct, and of those 

 that survive the typical, stalked forms (Fig. 436) live in deep water 

 and are less familiar than the shallow water feather stars {Antedon 

 and Actinometra) which, when they are adult, break off from their 

 stalks and swim by waving their arms. It will therefore be con- 

 venient to choose one of the latter to illustrate the anatomy of the 

 group. Antedon rosacea^ the common feather star, may be dredged 

 in ten fathoms of water on the coast of England. Its body is com- 



Fig. 433- Antedon bifida in oral view. From Sedg\vick, after Glaus. 

 A. anus ; M. mouth, 



posed of a small central region or calyx and five pairs of long, slender 

 arms, each bearing a double row of alternate branchlets known as 

 pinnules. On the convex aboral side, the calyx bears in the middle 

 a knob, formed by the centrodorsal ossicle, which is the stump of the 

 stalk and is fringed with numerous slender, jointed cirri, each ending 

 in a small hooked claw and used for temporary attachment. 



The flat top of the calyx is covered with a leathery tegmen, in the 

 middle of which is the mouth, while the anal papilla stands in one of 

 the interradii. Five amhulacral grooves start from the mouth, where 

 they are separated by five low triangular flaps, the oral valves, and 



